The six-cylinder turbodiesel in the 2014 Audi A6 TDI is the newest engine to power the A6, and it's putting up a clean fight. Its direct competitors are the diesel-powered BMW 535d xDrive and Mercedes-Benz E250 Bluetec 4Matic, but there's another interesting matchup. That car is one of Audi's own: the gas-powered, supercharged A6 3.0T, which was the first V-6 offered in the A6 lineup.

Shoppers looking for a six-cylinder A6 are likely pulling their hair deciding between the A6 TDI and A6 3.0T. If not, they should be. Both are excellent powerplants, and deciding between the two is a bit more complicated than choosing between gas and diesel or a turbo and a supercharger. As previously reported, Audi launched a diesel offensive in the U.S. for the 2014 model year, adding oil-burning powertrains to the A6, A7, A8, and Q5. More Americans are embracing clean and efficient diesels, and Audi's timing couldn't be better, given the upcoming CAFE regulations requiring automakers to again boost their average fuel economy.

That said, the 2014 A6 TDI's 3.0-liter V-6 turbodiesel, rated at 240-hp and 428 lb-ft of torque, is an excellent fuel-sipper. The EPA rated the engine at 24/38/29 mpg city/highway/combined. Meanwhile, the gas-powered A6 3.0T (310 hp and 325 lb-ft) is far thirstier at an EPA-rated 18/28/22 mpg. Both engines are mated to an eight-speed automatic that sends power to all four wheels via Audi's Quattro system.

When it comes to the A6 TDI's diesel rivals, the BMW and Mercedes are offered in both rear and all-wheel drive, but we'll stick with the latter to keep things simple. The 535d xDrive and its 3.0-liter turbocharged inline-six (255 hp and 413 lb-ft) is a tad stingier than the Audi at 26/37/30 mpg, while the 2014 Mercedes-Benz E250 Bluetec 4Matic trumps both German competitors thanks to its downsized 2.1-liter turbo-four (195 hp and 369 lb-ft) that gets 27/42/32 mpg.We had a little more than a week with the A6 TDI. And after a hard day at the track, countless hours slogging through L.A. traffic, and an easy, breezy road trip to San Diego, the A6 TDI returned 30 mpg. We logged 983 miles and only stopped for gas twice.

During our day at the track, we launched the 2014 A6 TDI from 0-60 mph in 5.4 seconds and through the quarter mile in 14.2 seconds. The 2012 A6 3.0T we tested posted times of 4.8 and 13.4 seconds, respectively. Around the figure eight, the diesel-powered A6 needed 26.1 seconds to complete the course, while its gas-powered sibling was slightly faster at 25.4. These numbers match the feeling from behind the wheel. Where the 3.0T is responsive almost to the point of being twitchy, the A6 TDI is slightly more subdued.

Still, a 5.4-second 0-60 time is impressive for a 4194-pound sedan, and the A6 TDI never felt lacking in power. Stomp on the accelerator and the engine responds with smooth and linear power delivery. Much credit goes to the eight-speed automatic, which is eager to shift and is tuned perfectly to the engine. The powertrain is quiet when it isn't being pushed hard. At times, the dancing tachometer is the only indication that the transmission is rowing though the gears.Despite our tester's large 20-inch, five-spoke rims and low profile tires, the cabin remained relatively quiet on the highway and rough L.A. roads. You can opt for smaller wheels to gain even more peace and quiet and save money. The 20s are an extra $1300, but they do turn heads (the big "TDI Clean Diesel" decals stuck on our tester's front doors may have attracted some attention, too). Enthusiasts will clamor for more steering feel, but the A6 TDI is fairly athletic and responsive. It never felt clumsy during a casual stint through some curvy back roads. Our tester's Drive Select system allows drivers to adjust steering resistance, suspension dampening, throttle response, and transmission shift points.Inside, the 2014 A6 TDI is still fairly handsome despite being almost four years old. The matte, open-pore wood garnered plenty of compliments, as did the clear infotainment screen. The instrument panel is one of the best in the segment, thanks to a large color information display that is clearly arranged and makes clever use of the space around the gauges.

Our tester was packed with options, and its as-tested price of $67,295 certainly reflects that. The Driver Assistance package is pricey at $2800, but adds adaptive cruise control, Pre Sense Plus collision prevention/readiness, lane keep assist, and a top view camera. While it's great for long and exhausting road trips and tricky parking spots, we'd skip this package. The Prestige package also costs $2800, but adds a number of niceties such as ventilated front seats, ambient lighting, and premium sound.That said, the A6 TDI carries a base price of $58,395 including $895 destination fee, which is $2400 more than the A6 3.0T. Furthermore, our partners at IntelliChoice gave the diesel-powered A6 an "above average" cost of ownership rating, while the A6 3.0T earned an "average" rating. Calculated using the car's base price, owning a 2014 A6 TDI for five years will cost a total of $64,817, versus $66,298 for the 3.0T. Both cars post similar numbers on most costs including insurance and repairs, but the TDI's obvious trump card was fuel costs.Summing up this sibling rivalry, then, is pretty simple. The A6 TDI goes farther on a tank of fuel, is just slightly more expensive than the 3.0T, and makes more financial sense in the long run. The A6 TDI might be slower than the 3.0T, but it doesn't fall too far behind and is still plenty entertaining from behind the wheel. Looks like we'll be seeing more TDI-badge A6 sedans on the road.

I don't think very many A6 buyers have much trouble choosing between the gas and diesel engines. My guess would be that 75% of consumers who drive out in a TDI walked in with the intention of buying the TDI. Salesmen may be able to coax some of the unenlightened into the diesel, but I think the vast majority of people are still skeptical of oil burners, not excited about them like many of us are.

I mean, if I had $60-70k to spend on a midsize sedan, choosing between the 3.0T and TDI would be the toughest part of shopping. I'm just saying that, in general, gas engines are still the top pick of Americans, especially when they're as good as Audi's gas engines.

On the street, the diesel's power curve makes much more sense. It is basically flat, unless you run 5500rpm on the street all the time, it will seem stronger.

And don't underestimate the fuel economy. That can be a whole week of commuting between fill ups, rather than doing it on cold, rainy, mid-winter, mid-week nights on the way home. It can also mean trips (e.g. NY-to-Boston or DC, round trip and driving around) on a single tank.

THe Audi's slow trap speed (under 95 MPH through the quarter, after such a quick 0-60 is pretty slow) shows it's lack of HP catches up with it on the highway in passing. Hurts you on the Autobahn, but will be plenty on slower American roads.....

I hope the newer A6s are more reliable. I once owned a 2000 A6 2.8 as well as an A4 2.8. After those two cars and the associated hassles with Audi ownership (frequent visits to the service department, poor reliability and expensive repair costs), I have never been tempted by Audis. Now, I stick to Volvos. Much more reliable.

I can't imagine why people are skeptical. Can you tell me why? I've driven huge F350s - they're awesome. Have driven light Hyundai's in Europe and they pull like frieght trains and idle-crawl up steeps slopes as soon as you pop them into first. Makes no sense to me why people are skeptical. Do they think they're smelly or maybe is it just hard to find stations that sell diesel or something?

Agreed about the refresh, though it'd still be my top pick for a premium midsizer. I may vacillate a bit between that and the XF, admittedly, just for the style statement the Jag makes. I don't like my cars to disappear in a crowd. Hence my Candy Blue Focus, rather than the gray everyone else in my office parking lot picked for their cars.

The 5-er and E-Class are still damn good cars. Maybe not from an enthusiast's point of view, but you can't deny their mass appeal. Though even Top Gear loves the current 5-Series, despite its panning at the hands of auto journalists in the U.S.

Yeah, I'm not sure MT realized how bad it sounded when they said they only filled up twice in 938 miles. My wife's 2008 Taurus can go 938 miles in two fill-ups easily, and it only gets 28 on the highway.

It's the AWD traction that gives them an edge off the line. But who cares? They still have amazing engines, and how often is the average buyer (or even one of us) going to go flat out from 0-100 against another $70k sedan?

Outpaced by what by the time they reach 100? Once you clear 30-40
mph, high revs become more valuable than high torque, at least in a
straight line race. A relatively low-revving diesel engine combined
with the additional AWD weight will definitely have a negative affect in
acceleration over 100 mph in comparison to a RWD gas-powered vehicle. If you're comparing the A6 TDI with the 535d and E-250 Bluetec, I doubt they will be outpacing the Audi past 100mph. Maybe the 535d with RWD would start to pull away above 100, but not if it also has AWD.

The
Jaguar XF is a beautiful-looking car. I think part of the appeal is
that you don't see as many of them on the road as you do an A6 or any other
Audi for that matter. That's somewhat because Jaguars have lower
resale value, therefore increasing lease payments and shutting out
prospective buyers that would otherwise want one.

You know torque is what matters when passing, not HP right? It's still plenty fast enough for pretty much any use, other than drag racing. Plus, as the guy above said, the diesel doesn't have to shift down 4 gears to pass at 70 mph.

I'd just like to pause for an anecdote. My cousin goes to Syracuse, and knew a Japanese exchange student who drove a GT-R, and put diesel fuel into it. When my cousin asked how he got the nozzle in, he said, "It wouldn't fit; I had to 'air' it in." F***in' brilliant. Needless to say, it wrecked the engine. Kid was so rich, he showed up in another one a week later.

I also drove my aunt's Land Rover once, to go fill it up, and she didn't think to tell me it took premium. Luckily, I'm such a car nerd I knew that already.

@Zack Kibler@Black Dynamite Online as for the diesel and gas motors for the A6 in the US, another
publication which shows 0 to 30, 40, ... posts better times for the gas
engine in all cases where the diesel is almost five seconds behind by 100. The 5-60 times for the diesel was 26% longer, the
30-50 passing time for the diesel was 33% longer and the 50-70 passing time for the diesel was 47% longer (these tests included turbo lag on the TDI which doesn't exist on the supercharged gas engine as well as any differences in the way the transmissions are geared and shift while already underway.) Having driven cars with both engines recently I wouldn't mind the TDI in the city but I'd definitely take the gas engine for the highway.

As for accelerating at passing speed, it's dependent on the components of horsepower (torque and engine RPM) as well as wheel RPM for the same size wheels. Ignoring assorted losses (driveline, tire, etc...) the linear force a car will exert on the pavement is going to be the torque in pound-feet divided by the radius of the drive wheels in feet resulting in the force in pounds, assuming the engine is directly connected to the wheels. To account for the gearing of the transmission which act as a lever you have to multiply the radius of the wheels by the ratio of the wheel RPM to engine RPM. Multiply the numerator and denominator of the first equation by the reciprocal of the second and you get:

Force = ( Torque * ( EngineRPM / WheelRPM ) ) / WheelRadius

Given the same torque, wheel RPM and wheel radius more horsepower will result in better acceleration as horsepower proportional to torque times engine RPM and torque was defined to be the same in this example. In some cases lower torque can still be overcome with higher engine RPM. Note that neither torque nor horsepower are constant across the engine RPM range so you can't just look at peak numbers and know how the car will behave.